Italy's first spacewalker, Luca Parmitano, reported that he felt a lot of water on the back of his head barely an hour into the mission.

The leak was so bad that he needed help from a fellow astronaut getting back into the safety of the space station.

Mr Parmitano at first thought it was sweat because of the exertion of performing routine cable work, but he was repeatedly assured it was not sweat. His spacewalking partner, American Christopher Cassidy, said it might be water from his drink bag. Mr Cassidy said it looked like half a litre of water leaked out.

The water eventually got into Mr Parmitano's eyes, prompting Nasa to order the two men back inside. "It's a lot of water," said Mr Parmitano once he was back in the air lock of the International Space Station.

The trouble cropped up about 90 minutes into what was to be a six-hour spacewalk to perform cable work and other routine maintenance. It was the astronauts' second spacewalk in eight days.

Mr Parmitano startled everyone when he announced that he felt a lot of water on the back of his head. He could not speak as the water drenched his nose and mouth, and he had trouble hearing on the radio lines. "He looks fine," Mr Cassidy assured everyone. "He looks miserable. But okay." Mr Cassidy quickly cleaned up the work site once Mr Parmitano was back in the air lock.

Nasa rarely cuts a spacewalk short. It was the fastest end to a spacewalk since 2004 when Russian and American spacewalkers were ordered back in by Mission Control outside Moscow because of spacesuit trouble. That spacewalk lasted 14 minutes. This was the second spacewalk for Mr Parmitano, 36, a former test pilot and Italian air force officer. He became the first Italian to conduct a spacewalk last Tuesday, more than a month after moving into the space station. Mr Cassidy, 43, a former Navy Seal, is a veteran spacewalker midway through a six-month station stint.

Mr Parmitano looked fine on Nasa TV as he gestured with his hands to show his crewmates where the water had crept over his head. Mr Cassidy told Mission Control in Houston: "To him, the water clearly did not taste like our normal drinking water." A smiling Mr Parmitano then chimed in: "Just so you know, I'm alive and I can answer those questions too."

Mission Control praised the crew for its fast effort and promptly scheduled a radio hook-up with flight surgeons on the ground. Engineers, meanwhile, are scrambling to determine the source of the leak.